Silica long has been used, either per se or coated with an organic material, as a stationary phase in chromatography, and as such has enjoyed broad success and applicability. As new chromatographic needs arose, these often were met by changing the properties of silica, so that several discrete kinds of silica have been used as a stationary phase. For example, Okamoto and his group recently have described chiral stationary phases, i.e., stationary phases coated with a chiral organic material and used in the chromatographic separation of racemic mixtures to afford chiral components, and has found that such separations are effected particularly well using large-pore silica as a support, i.e., silica having pores on the order of 300-1,000 angstroms. See, for example, Y. Okamoto and Y. Kaida, J. Chromatography A, 666 (1994) 403-419.
Silicas containing large pores are available, but routes to their preparation result in high cost, low reproducibility, and limited availability. For example, a pore-filling/melt procedure has been successfully used to prepare large-pore silica with pore sizes 1,000 angstroms and greater. In this procedure silica is impregnated with a salt, such as sodium chloride, by an incipient wetting technique, so the pores are filled with a solution of sodium chloride. The wetted materials are carefully dried, then heated to a temperature where sodium chloride liquefies and the pore structure of silica is disrupted. Upon cooling, the silica recrystallizes around the salt, generating pores within the bulk silica of large size (.gtoreq.1,000 angstrom pores). Subsequently the salt is washed out to leave silica having the aforementioned large-pore structure.
It has been observed that the silicas prepared by the foregoing method have a bimodal pore distribution. We also have observed that the silicas resulting from the aforedescribed procedure have little, if any, silanol functionality, which we believed would be important where silica is used as a support for an organic coating. The aforegoing observation is reasonable given the high calcination temperatures used. Last, the method is difficult to apply in making commercial sized batches.
What we believed necessary as a support for CSPs was a silica having large pores (at least 1,000 angstroms) with a narrow pore size distribution, a relatively low surface area (20-30 meters per square gram (m.sup.2 /g)) and particles at least within the 50-150 micron range to accommodate commercial-size separations, particularly by simulated moving bed chromatography. It was required of any method of making such materials that the method be readily adaptable to commercial-size runs, that it afford control over pore size so as to give a reproducible pore size and distribution, that it was applicable to silicas generally, and that it was relatively inexpensive and did not require specialized equipment or a severe heat treatment. It was further required that such material be an acceptable support for a broad spectrum of chiral organic materials currently used as the coating in a CSP; in short, we sought a silica which could be substituted for existing silicas to give a CSP at least as effective as prior art materials.
What we have found is that when silica is contacted with a mineralizing agent in the temperature range of 85.degree.-300.degree. C. for a time as short as four hours and up to several days, one introduces into the silica large pores with a unimodal size distribution. This offers a general route to large pore silicas whose pore size can be readily controlled; the preparative method can be practiced effectively without specialized equipment, avoids the high temperatures of prior art methods, and is readily practiced on a large, commercial scale. We have further observed that CSPs based on such silicas as the support often function even more effectively than CSPs made with prior art silicas and provide more cost-effective separations because of the more economical support.